Coding Across Time in Quantized Feedback Systems

1
Coding Across Time in Quantized Feedback
Systems Che Lin, Vasanthan Raghavan, and
Venugopal V. Veeravalli
Illinois Center for Wireless Systems

• 0. Motivations
• Advantage of MIMO systems can be fully exploited
  only when there is perfect channel state
  information (CSI) at the transmitter.
• Next generation wireless systems often utilize
  the reverse link feedback to convey CSI to the
  transmitter to enhance overall performance.
• Codebook-based feedback systems have been
  adopted in latest standards, e.g. 802.11n,
  802.16e, and 3GPP-LTE etc..
• If the channel varies slowly enough, how can one
  design a good codebook that accounts for the
  redundancy in time?

• 4. Properties of Optimal LD Codes
• Uniform code covariance matrices is optimal for
  any amount of channel
• knowledge at the transmitter.
• When the transmitter has access of only
  statistical information, optimal spatial power
  allocation excites multiple modes in general.
• Optimal spatial power allocation is of rank-one
  when there is perfect CSIT.
• Initial research suggests that optimal spatial
  power allocation is also of rank-one when there
  is any amount of CSI feedback.

• 6. Coding Across Time
• With partial CSIT, space-time code can have two
  components one that evolves with the channel,
  and the other that does not.
• Either component can evolve with time in the
  most general case.
• In this context, we say that the space-time code
  does not code across time if and only if the
  component that evolves with channel is
  independent of time.

• 9. Conclusions
• Uniform code covariance matrices is optimal with
  any amount of CSIT.
• With any amount of CSIT, rank-one codebooks are
  optimal.
• Rank-one space-time codes do not code across
  time and greatly reduce complexity
• for implementation.
• Therefore, the optimal codes do not code across
  time even with 1 bit of feedback.

1 C. Lin, V. Raghavan, and V. Veeravalli,
Optimal Power Allocation for Linear Dispersion
Codes over Correlated MIMO Channels with Channel
State Feedback, to appear in Globcom 07. 2 C.
Lin and V. Veeravalli, A Limited Feedback Scheme
for Linear Dispersion Codes over Correlated MIMO
Channels, to appear in ICASSP 07, Honolulu,
Hawaii, Apr. 2007. 3 C. Lin and V. Veeravalli,
Optimal Linear Dispersion Codes for Correlated
MIMO Channels, to appear in IEEE Trans. Wireless
Communication.